Determination of firmness and sugar content of apples using near-infrared diffuse reflectance

The objective of this research was to study the potential of near-infrared (NIR) diffuse reflectance between 800 nm and 1700 nm for determining the firmness and sugar content of apples and to ascertain the effects of apple peel and variety on the NIR prediction of these two quality attributes. The spectral reflectance data were acquired from both peeled and unpeeled 'Empire', 'Golden Delicious', and 'Red Delicious' apples. Statistical models were developed using principal component analysis/regression. Lower correlations of prediction were obtained (r=0.38 to 0.58) between NIR measurement and Magness-Taylor firmness for both unpeeled and peeled fruit, with the standard errors of prediction (SEP) between 6.6 N and 10.1 N. Improved predictions were obtained when NIR reflectance was correlated with the slope of the Magness-Taylor force-deformation curves. Excellent predictions of the sugar content in peeled apples were obtained (r=0.93 to 0.97; SEP=0.37 to 0.42 degrees Brix). The SEP, on average, increased by about 0.17 degrees Brix for the unpeeled apples. Variety did not have a large effect on the model performance on sugar content predictions.